1. Field of the Invention
The present invention is related to the field of electric wireline tools used to withdraw samples of fluids contained within pore spaces of earth formations. More specifically, the present invention is related to systems for determining various fluid flow properties of earth formations by using a formation testing apparatus having a plurality of fluid sampling probes which are radially and axially spaced apart and hydraulically isolated from each other.
2. Description of the Related Art
Electric wireline formation testing tools are used to withdraw samples of fluids and to make pressure measurements of fluids contained within pore spaces of earth formations. Calculations made from these measurements can be used to assist in estimating the total fluid content within the earth formations.
Formation testing tools known in the art are typically lowered at one end of an armored electrical cable into a wellbore drilled through the earth formations. The formation testing tools known in the art can include a tubular probe which is extended from the tool housing and is then impressed onto the wall of the wellbore. The probe typically is externally sealed by an elastomeric packing element to exclude fluids from within the wellbore itself from entering the interior of the probe as fluids are withdrawn from the earth formation through the probe. Various valves selectively place the probe in hydraulic communication with sample chambers included in the tool. The probe can also be connected to a highly accurate pressure sensor which measures the fluid pressure at or near the probe. Other sensors in the tool can make measurements related to the volume of fluid which has entered the sample chambers during a test of a particular earth formation. The formation testing tools known in the art can also include a sample tank. The sample tank can be selectively connected to the probe so that a quantity of fluid withdrawn from the formation can be discharged into the sample tank and transported to the earth's surface for laboratory analysis.
Other formation testing tools known in the an can include more than one probe. For example, one formation testing tool known in the art includes two collinear probes positioned at axially spaced-apart locations along the tool. By providing two probes at axially spaced apart locations, it is sometimes possible to determine to what extent a particular earth formation has permeability coaxial with the wellbore. Typically, one of the two probes in the two-probe tool is used to withdraw fluid from the formation while monitoring fluid pressure at the tither probe. The time elapsed between withdrawal of the fluid at the one probe and indication of pressure drop at the other probe can be indicative of the coaxial permeability of the earth formation.
A drawback to the two-probe tool known in the art is that it is unable to resolve permeability discontinuities which may cross the wellbore at certain oblique angles. Using the two-probe tool known in the art, it is possible that coaxial permeability discontinuities which may be observed with the tool in one rotary orientation within the wellbore may not be observed in other rotary orientations, which allows the possibility that coaxial permeability discontinuities of significant interest to the wellbore operator could go undetected.
It is also known in the art to provide a formation testing tool having two probes opposingly faced and located at substantially the same axial position along the tool in addition to the axially spaced apart collinear probes. The opposingly faced probes can observe some permeability discontinuities intersecting the wellbore obliquely which may be missed by the axially-spaced apart probes. Such a tool is described for example in U.S. Pat. No. 5,335,542 issued to Ramakrishnan et al.
A drawback to the tool in the Ramakrishnan '542 patent having opposingly faced probes is that this tool may provide insufficient radial resolution to observe permeability discontinuities which may traverse the wellbore in such a way as to make the apparent permeability substantially equal as observed by either opposing probe relative to the axially spaced-apart probe.
A still further drawback to the formation testing tools known in the art is that the probes used to withdraw fluid samples typically have small cross-sectional areas relative to the surface area of the wellbore. Some features of earth formations which can be highly productive of oil and gas may intersect only a very small portion of the surface area of the wellbore and there wellbore have a high probability of being missed by one of the probes on the formation testing tools known in the art. Such features can include fractures or thin layers of permeable sandstone interleaved with impermeable strata such as shale.
It is known in the an to provide a means for isolating a substantial axial section of the wellbore so that the entire surface area of the wellbore within the section can be exposed to fluid withdrawal by a formation testing tool. Axial sections can be isolated by providing a device known as a straddle packer. The straddle packer known in the an includes two inflatable elastomeric bladders positioned at axially-spaced apart locations along the tool. A port is provided on the tool at an axial position in between the bladders. The port can be selectively hydraulically connected to the various sample chambers of the formation testing tool. As it is typically used, the straddle packer is positioned within a zone of interest, the bladders are inflated to hydraulically isolate the zone and fluid is withdrawn through the port by various pumping and flow control devices in the tool.
A drawback to the straddle packer is that the bladders can only isolate the zone of interest axially. The straddle packer is unable to provide measurements determining permeability coaxial with the wellbore or for determining the presence of coaxial permeability discontinuities intersecting the wellbore. Further, the large volume which is isolated between the bladders results in a large volume of fluid that must be withdrawn from the axial section bladder native fluid from the formation enters the testing tool. Withdrawing a large fluid volume can require leaving the tool in place for a long time. Leaving the tool in place for a long time can be unsafe and expensive. Further, the capacity of the fluid pumps in formation testing tools known in the art is limited. It can be difficult to determine the permeability of highly permeable formations using the straddle packer tool known in the art, because the large surface area of the wellbore which is exposed to fluid withdrawal can provide a high volume of fluid relative to the volume that the pump is capable of withdrawing. If the formation can produce fluid faster than the fluid can be pumped away, then substantially no pressure drop will occur. To determine permeability requires at least some amount of pressure drop from the earth formation's original pressure to be measured.
It is an object of the present invention to provide an electric wireline formation testing tool which can provide improved radial resolution of permeability discontinuities intersecting the wellbore.
It is a further object of the present invention to provide a formation testing tool which can withdraw fluid from permeable features intersecting the wellbore which have a small surface area, while reducing the volume of fluid tram within the wellbore which must be pumped away before sampling of the native fluid can begin.
It is yet a further object of the present invention to provide a formation testing tool which can withdraw fluid from permeable features intersecting the wellbore which have a small surface area, while maintaining the ability to determine permeability of the formation even if the permeability is very high.